Back to top

Can A Horse Stay in Heat?

For a horse owner with mares, knowing and understanding your mares’ heat cycle is an essential part of equine management. 

When a mare stays in heat constantly and shows signs of erratic heat cycles, there is usually a medical reason to blame for this abnormal behavior. The presence of an ovarian cyst, uterine infection, or hormonal imbalance is most probably be the underlying cause of your mare staying in heat all year round.

Health care is extremely important for a cycling mare. Let’s have a look at how to diagnose underlying health issues and what treatment options there are for a mare that constantly displays signs of heat.

What is the regular heat cycle of a mare?

Depending on where you live, if you are in the Nothern hemisphere, mares will come into season from April to October. In the southern hemisphere, it will range from October to April. Most mares are seasonal breeders and come into heat when the days are long. Reproductive receptiveness is triggered by the longer hours of daylight that begin hormone production.

A typical heat cycle of a mare consists of 7 days of estrus and 14 days of diestrus. A mare will ovulate every 21 days during the spring and summer breeding season. During this time, her estrus (sexual receptivity) can vary between 2 to 8 days.

The length of diestrus will vary accordingly to maintain the 21-day interval. At the beginning of the breeding season, estrus tends to be longer but will reduce to around 2 to 3 days around the summer solstice. In winter, when the sunlight hours begin to shorten, mares will start to go into an anestrous cycle and not show any signs of heat again until the next season.

What are the signs of a mare in season?

Mares will show they are in heat by squatting, urinating, lifting their tail, squealing, frequent posturing, and exposing their vulvar lips to entice copulation. Irritability, lack of focus, reluctance to work, aggressive biting, and kicking are just some of the behaviors a mare will exhibit when in heat. When in diestrus, mares will usually kick, bite and reject a stallion’s advances.

What to do when your mare is in heat

Firstly, you don’t really have to do anything. Coming into heat is a pretty standard thing for your mare, and you shouldn’t have to worry about it at all. However, there are a few tips to help you deal with a moody horse and how to keep your mare feeling comfortable. 

Ovulation can be painful; your mare will benefit from some Banamine or a small dose of Phenylbutazone (Bute) for pain relief. Usually, a day or two of pain relief will make a world of difference. Take note Phenylbutazone cannot be administered in horses that are competing. 

Be careful not to approach your mare from behind; remember her irritability is extreme, and she may lash out at you. 

When you are grooming your mare, start at her neck and shoulders before you begin grooming at her sensitive flanks. 

Don’t punish your horse for being moody and irritable. Even though it can be frustrating for you, understand that your horse is feeling cranky and uncomfortable.

Reasons why a mare could stay in heat

When a mare shows signs of constantly being in heat, there could be some underlying medical conditions that are the reason for her to stay in heat all the time. Some of these could be an ovarian cyst or a tumor. Hormonal changes are also often a cause for staying in heat. 

Call your vet and schedule a full health check and an ultrasound scan of your mare’s reproductive system and ovaries and discuss blood tests to detect hormonal imbalances. 

Hormone imbalance

Hormones travel through the bloodstream delivering messages to the organs and glands in the body. They regulate body functions and metabolism. Hormones can also influence behavior. Too little or too much of any hormone can cause imbalances that can affect your horse’s behavior and health. 

The endocrine glands like the ovaries, testes, thyroid, and pituitary glands produce most of the body’s hormones that the body needs to function. Hormones associated with estrus, like estrogen, act on the behavioral part of the brain and may cause some of the symptoms.

Hormone therapies can help and are available on prescription from your veterinarian. Discuss the options with your veterinarian to find the treatment plan that works for your mare.   

Ovarian cysts

Ovarian cysts can cause your mare to stay in heat all year round. Ovarian cysts cause a fluctuation of hormones that will affect your horse’s reproductive system resulting in erratic heat cycles. 

Some mares show signs of frequent heat, and some mares cease to cycle at all due to the mixed signals sent from improper hormone production. 

Your veterinarian will be able to rule out any ovarian cysts with a rectal exam, hormonal testing, or ultrasound and prescribe any medications and treatments if necessary. 

Granulosa cell tumor

These tumors are the most common tumors found in mares and often affect middle-aged mares and sometimes have a slow onset. Typically, one ovary is affected; in some cases, both ovaries can be affected. 

Mares that suffer from granulosa cell tumors display signs of aggression. Estrus can be significantly prolonged, irregular, or completely absent. Your mare can also show signs of colic.

Usually, the affected ovary is more prominent with a multicystic appearance than the other ovary is most often inactive and small due to the hormones excreted from the affected ovary. 

Your veterinarian can make a diagnosis by ultrasonography. Diagnosed granulosa cell tumors are removed surgically.  A laparoscopy is more commonly used to remove a granulosa cell tumor. 

Mares that have undergone this procedure where one of the mare’s ovaries was removed have a good prognosis. In 6 to 8 months, your mare should start cycling regularly and would be capable of conceiving and foal normally.      

Uterine infections

Some mares have difficulty in conceiving and are often diagnosed with Chronic Uterine infection (endometritis).  Endometritis causes poor fertility and can cause a mare to short cycle herself and come into heat every 5 to 7 days. 

This shortens diestrus dramatically, and the interval between estrus may seem like the mare is constantly staying in heat.

Your veterinarian can do an ultrasound scan of the reproductive tract to diagnose any free fluid in the lumen of the uterus in your mare. Endometrial swabs and laboratory tests may be necessary to identify an infection.  Daily antibiotic lavage treatment into the uterus will be required.  

Herbal treatment for moody mares

The most commonly used treatments used for hormonal mares are herbal supplements. These supplements contain mixtures of plants and herbs that act on the hormonal system and have a mildly sedative effect on horses.

Try a few supplements to see which one works best for your mare. The chaste tree berry (a common ingredient) is thought to act on the pituitary gland to suppress seasonal cycling.

If you are competing with your mare, make sure to check that the ingredients are not on the FEI-controlled substance list. 

The horse “pill” option

When your mare is constantly in heat, an oral hormonal treatment that simulates progesterone (released during pregnancy) that prevents estrus can be given to your mare as a treatment. 

The commercial product, Regumate Equine, is most commonly used to suppress estrus in mares. Regumate has caused temporary infertility in some mares that have lasted even after treatment has stopped. 

Mares that were given Regumate and, when they stopped, had cycling abnormalities for months afterward. This can exaggerate the irregular cycles and make things seem even worse. 

Another critical point to remember with Regumate is to take care not to let it touch your skin where it can be absorbed. Regumate can cause cycling problems for women when it gets into your system and cause infertility in both women and men.

Under FEI regulations, this product may be used in competing horses. 

A new treatment option recently developed in the US comes in the form of a long-acting progestagen injection (that mimics progesterone) that can be administered once every two weeks.

There is currently research ongoing to develop an anti-GNRH (reproduction hormone) vaccine that aims to prevent estrus. This research is still in the experimental stage but might offer hope to mares suffering from being in constant heat. 

The key is to find the appropriate long-term medical and management treatment for your mare. Giving your mare time to rest during estrus is also a good option.


A mare that is displaying excessive estrus behavior all year round is not exhibiting normal reproductive behavior, and there is usually an underlying health problem like granulosa cell tumor, uterine infection, or a hormonal imbalance to blame. 

Even though this can be very frustrating to deal with, thankfully, there are many treatment options available to treat these disorders. Your veterinarian will be able to advise and suggest and administer the appropriate diagnostic treatment for your mare.    

Anrie Diedericks

I've been around horses since I was 6 years old and started competing at the age of 9. Horses are my greatest passion and I am thrilled to be able to share my 23 (and counting) years of experience and knowledge with you.

Recent Posts